There are a number of situations when using a flash outdoors in daylight is helpful. Its not mandatory, nor essential but can often help out a photo.

If the sun is high (approx 11am to 3pm) and the person you are photographing is wearing a hat (like a ball cap) there will be a shadow on their eyes. You can use flash to lighten up the shadow, making their eyes more legible.

There may also be scenes without people with shadows you want to lighten up with flash. The trick is to use it carefully, subtly, so it doesn’t look over lit and fake.

If you are photographing a person who is standing in shade, but the background is brightly lit, you may have a difference in exposure of 3-4 stops between the person and the background. You can use flash to narrow the gap, close the difference in exposure, and narrow the difference in contrast.

With outdoor portraits adding a flash outdoors produces a catch light in the eye. A catch light is the little sparkle of brightness in the subjects eyes, caused by the flash. It is a nice feature and gives the viewer a precise point to focus on when looking at the photo.

Flash power, no matter how much you dial it up will NOT usually overpower the sun, unless youre using a large powerful, studio type unit. That means you should be using the flash to supplement the sun, not trying to use it to provide the main light.

Determine the throw distance of the flash, how far the flash will work, the effective range of the flash. Typically this is only 10 feet on a little pop up in-camera flash, and maybe 20-25 feet for an external on-camera flash. Anything further away and the flash really wont do anything.

Change the flash power, the output. To control the output of your flash use the +/- flash power adjustment and a little trial and error. This is usually available in the camera menu. The same feature is available on many external flash models. Determine your ambient exposure first (the regular exposure without using flash). Get a good exposure without flash and then add a low power flash to see the result. You then dial the flash power up or down incrementally to get the effect you want.

Both photographs here are shot outdoors with fill flash which brighten up shadows and give the photo a bit more crispness and sharpness. The colors pop more with a little fill flash during the day.